With reference to the unusual RTF phraseology,
Presumably the authorities have listened to all the messages/acknowledgements from that night's MH370, therefore it should not be hard to determine whether the final transmissions were made by either of the two men who were rostered to fly the a/c.
ATC tapes are kept for a few weeks before being overwritten, so it would not be hard to find other recordings of flights where either of the crew were doing the RTF and, if necessary, subject those tapes to voice-spectral analysis..